List problem

[Peter Hansen]

Reinhold Birkenfeld wrote:
> Alex Martelli wrote:
>>    a[:] = [x for x in a if x != 2]
> 
>        ^^^
> What are these for?

Observe the difference between these two approaches:

Python 2.3.4 (#53, May 25 2004, 21:17:02) [MSC v.1200 32 bit (Intel)]
 >>> b = a = range(5)
 >>> b is a
True
 >>> a[:] = [x for x in a if x != 2]
 >>> a
[0, 1, 3, 4]
 >>> b
[0, 1, 3, 4]
 >>> b is a
True

 >>> b = a = range(5)
 >>> a = [x for x in a if x != 2]
 >>> b is a
False


[:] on the left side of the assignment basically causes
"slice assignment" to the whole list, modifying it in-place
instead of creating a new list.

-Peter